System comprising an integrated waveguide-coupled optically active device and method of formation

ABSTRACT

Integrated-optics systems are presented in which an optically active device is optically coupled with a silicon waveguide via a passive compound-semiconductor waveguide. In a first region, the passive waveguide and the optically active device collectively define a composite waveguide structure, where the optically active device functions as the central ridge portion of a rib-waveguide structure. The optically active device is configured to control the vertical position of an optical mode in the composite waveguide along its length such that the optical mode is optically coupled into the passive waveguide with low loss. The passive waveguide and the silicon waveguide collectively define a vertical coupler in a second region, where the passive and silicon waveguides are configured to control the distribution of the optical mode along the length of the coupler, thereby enabling the entire mode to transition between the passive and silicon waveguides with low loss.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is claims priority to U.S. Provisional Patent Application Ser. No. 62/961,348, filed Jan. 15, 2020, entitled “System Comprising an Integrated Waveguide-Coupled Optically Active Device and Method of Formation,” which is incorporated by reference. If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case.

TECHNICAL FIELD

The present disclosure relates to heterogeneous integration of compound-semiconductor structures on silicon substrates in general, and, more particularly, to the integration of compound-semiconductor active and passive photonic elements with silicon for forming semiconductor waveguides, active electronic devices, and/or active and passive photonic devices.

BACKGROUND

Silicon photonics promises relatively low-cost solutions for many photonic applications. However, historically, most silicon-photonics systems cannot generate light on the chip. As a result, there are ongoing efforts toward enabling on-chip light generation, amplification, modulation, etc. via heterogeneous silicon-photonic integration.

Some success in providing on-chip light generation, amplification, modulation, etc., in silicon-photonics systems has been achieved by integrating compound-semiconductor material onto a substrate containing silicon waveguides via direct bonding and subsequent formation of active compound-semiconductor waveguides above silicon waveguides.

In some alternative prior-art approaches, discrete optically active devices (e.g., lasers, amplifiers, modulators, etc.) are completely formed separately and optically coupled with a photonic integrated circuit (PIC) through, for example fiber coupling the device and the PIC, flip chip bonding the device onto the PIC substrate and optically coupling it with a silicon waveguide on the PIC via a silicon grating coupler, or employing conventional packaging methods wherein light is coupled between the discrete device and a silicon waveguide on the PIC.

Unfortunately, the cost and complexity of such approaches have, thus far, limited their use.

A platform that enables cost-effective formation of a heterogeneous silicon photonic system in a cost-effective manner remains, as yet, unmet in the prior art.

SUMMARY

The present disclosure is directed toward integrated-optics systems that include an optically active device optically coupled with a silicon waveguide via a passive compound-semiconductor waveguide, where the optically active device, the passive waveguide, and the silicon waveguide reside on a common substrate. Embodiments in accordance with the present disclosure are particularly well suited for use in distributed-feedback lasers, mode locked lasers, photonic integrated circuits, external-cavity mode locked lasers, loop-mediated isothermal amplification devices, and the like.

An advance is made over the prior art by employing a passive compound-semiconductor waveguide as a transition element between an optically active device and a silicon waveguide. The optically active device includes active material that is disposed on and optically coupled with a compound semiconductor coupling waveguide. The active material resides completely in a first region of a substrate. The silicon waveguide is wholly contained in a second region of the substrate. The passive waveguide resides in a transition region between the first and second regions and extends into these regions such that the passive waveguide is optically coupled with each of the optically active device and silicon waveguide. The active material, the coupling waveguide, the passive waveguide, and the silicon waveguide configured to dictate the vertical location and lateral confinement of optical energy at each point along the length of the system.

The use of a compound-semiconductor passive waveguide as a transition element to optically couple an optically active device and a silicon waveguide affords significant advantages over the prior art, such as:

-   -   i. independent control over the performance of the optically         active device and the coupling efficiency of a light signal into         the silicon waveguide, thereby enabling each to be substantially         optimized without degrading the other; or     -   ii. transition of a light signal between the optically active         device and the silicon waveguide with less optical loss than can         be achieved in the prior art; or     -   iii. in monolithically integrated systems, facilitated epitaxial         growth of the optically active device by virtue of the presence         of a compound semiconductor layer from which the passive         waveguide is formed in the first region;     -   iv. in heterogeneously integrated systems, facilitated bonding         of nascent optical-device material on the common substrate by         virtue of the presence in the active region of a compound         semiconductor layer, from which the passive waveguide is formed;         or     -   v. any combination of i, ii, iii, and iv.

Further advance over the prior art is made by defining a coupling waveguide and a passive waveguide from contiguous portions of a compound-semiconductor layer and forming a composite waveguide having a rib portion that is an optically active device and a planar portion that is the coupling waveguide. By tailoring the lateral dimensions of the optically active device, the size and/or vertical position of an optical mode of a light signal in the composite waveguide can be controlled. As a result, the optical mode can be located at least partially in the optically active device at one location and forced substantially completely into the passive waveguide as the light signal propagates to a second location.

An illustrative embodiment is a waveguide-coupled optical system having an active region, an output-coupling region, and a transition region, each of which is disposed on a silicon-on-insulator substrate. The system includes: a quantum-dot laser whose active material is located completely within an active region, where it is disposed on and optically coupled with a gallium arsenide coupling waveguide; a silicon waveguide that resides completely in the output-coupling region; and a passive gallium-arsenide waveguide that resides in the transition region and extends into each of the active and output-coupling regions where it is optically coupled with the laser and the silicon waveguide, respectively. As a result, the passive waveguide couples optical energy generated by the laser into the silicon waveguide.

In the active region, the active material is patterned to define a tapered region that defines a first coupler that facilitates transfer of optical energy generated in the laser structure into a light signal propagating in the coupling waveguide.

In the output-coupling region, the silicon device layer of the silicon-on-insulator substrate is patterned to define a silicon waveguide that functions as a single-mode waveguide for the light signal.

In the transition region, the coupling waveguide is patterned to define a single-mode passive waveguide for the light signal. No optically active material or electrical contacts are present in the transition region. The passive waveguide extends slightly into the output-coupling region, where it is tapered to define a second coupler that facilitates transfer of the light signal into the silicon waveguide.

In some embodiments, the coupling layer includes a lower sub-layer and an upper sub-layer, where the lower sub-layer is doped to facilitate forming electrical contacts. The upper sub-layer is characterized by a higher refractive index than the lower sub-layer, thereby enabling the lower sub-layer to also function as a lower cladding layer.

In some embodiments, the optically active device is a device other than a laser, such as an optical modulator (e.g., an electroabsorption modulator, a phase modulator, etc.), an optical amplifier, a variable optical attenuator, a photodetector, and the like. In some embodiments, the optically active device includes a quantum element other than a quantum dot, such as a quantum dash, quantum well, a quantum wire, and the like. In some embodiments, optically active device does not include a quantum element.

In some embodiments, the active region includes an optically active device and coupling layer of a different compound semiconductor, such as indium phosphide, indium gallium arsenide phosphide, and the like. As a result, the passive waveguide also comprises this different compound semiconductor.

In some embodiments, a reflector is defined in at least one of the passive and silicon waveguides to redirect a light signal. In some embodiments, the reflector is defined in the passive waveguide and is configured to optically couple with a vertical grating coupler defined in the silicon waveguide.

In some embodiments, an alignment feature is included for passively aligning a bulk optical element (e.g., an optical fiber, a photodetector, a light source, etc.), to one of the passive and silicon waveguides. In some such embodiments, this alignment feature is a silicon-optical-bench feature.

In some embodiments, the silicon waveguide is not included and the passive waveguide functions as an optical interface to another optical element. In some embodiments, the other optical element and the optically active device are disposed on the same substrate. In some embodiments, the other optical element is external to the substrate comprising the optically active device. In some such embodiments, the passive waveguide is configured such that its optical mode is substantially matched with the optical mode of an external element to mitigate optical coupling loss.

In some embodiments, a spot-size converter is included in the passive waveguide and/or the silicon waveguide to facilitate optical coupling with an external element.

An embodiment in accordance with the present disclosure is an integrated-optics system disposed on a substrate, the system comprising an optically active device that is optically coupled with a silicon waveguide, the system including: a first region that selectively includes the optically active device, the optically active device being disposed on a coupling waveguide that includes a first portion of a first layer that comprises a compound semiconductor, wherein the optically active device and the coupling waveguide collectively define a composite waveguide that at least partially supports an optical mode of a light signal; a second region that includes a first waveguide that is a passive waveguide configured to at least partially support the optical mode, the first waveguide having a first core that comprises a second portion of the first layer, the first waveguide being optically coupled with the composite waveguide; and a third region that selectively includes a second waveguide having a second core comprising single-crystal silicon, wherein the second waveguide is configured to at least partially support the optical mode and is optically coupled with the first waveguide; wherein the first region, second region, and third region are contiguous.

Another embodiment in accordance with the present disclosure is a method for forming an integrated-optics system disposed on a first substrate, the system having a first region, a second region, and a third region and comprising an optically active device that is optically coupled with a silicon waveguide, the method including: forming a coupling waveguide in the first region, the coupling waveguide including a first portion of a first layer comprising a compound semiconductor; forming an optically active device on the coupling waveguide, the optically active device comprising at least one quantum element, wherein the optically active device and the coupling waveguide collectively define a first composite waveguide that at least partially supports an optical mode of a light signal, and wherein the optically active device is located only in the first region; forming a first waveguide in a second region, the first waveguide being a passive waveguide configured to at least partially support the optical mode, wherein the first waveguide has a first core that comprises a second portion of the first layer, and wherein the first waveguide is optically coupled with the first composite waveguide; and forming a second waveguide in a third region, the second waveguide having a second core comprising single-crystal silicon and being configured to at least partially support the optical mode, wherein the second waveguide is optically coupled with the first waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of a top view of an illustrative integrated-optics system in accordance with the present disclosure.

FIGS. 2A-E depict sectional views of system 100 through lines a-a through e-e, respectively.

FIG. 3 depicts operations of a method suitable for forming system 100 in accordance with the illustrative embodiment.

FIG. 4A depicts a cross-sectional drawing of a portion of a sacrificial substrate comprising coupling layer 208 and active-material stack 210.

FIG. 4B depicts a cross-sectional view of nascent system 100 after coupling layer 208 and silicon device layer 206 have been bonded.

FIG. 5A depicts a schematic drawing of a sectional view of an alternative composite waveguide in accordance with the present disclosure.

FIG. 5B depicts a schematic drawing of a sectional view of another alternative composite waveguide in accordance with the present disclosure.

FIG. 6 depicts a schematic drawing of a sectional view of an alternative transition region in accordance with the present disclosure.

FIG. 7 depicts a schematic drawing of a sectional view of an alternative embodiment of a coupler for optically coupling a passive waveguide and a silicon waveguide in accordance with the present disclosure.

FIG. 8 depicts a schematic drawing of a cross-sectional view of yet another alternative coupler in accordance with the present disclosure.

FIGS. 9A and 9B depict schematic drawings of cross-sectional views of alternative output ports in accordance with the present disclosure.

FIG. 10 depicts a schematic drawing of an alternative system in accordance with the present disclosure.

FIG. 11 depicts a schematic drawing of another alternative system in accordance with the present disclosure.

DETAILED DESCRIPTION

For the purposes of the present disclosure, including the appended claims, the following terms are defined:

-   -   “disposed on” or “formed on” is defined as is defined as “exists         on” an underlying material or layer with or without intermediate         layers. For example, if a material is described to be “disposed         (or grown) on a substrate,” this can mean that either (1) the         material is in intimate contact with the substrate; or (2) the         material is in contact with one or more layers that reside on         the substrate.     -   optically active device is defined as an electrically coupled         device (i.e., a device comprising electrical contacts for         connecting to external circuitry) in which, in response to an         electrical signal applied to the electrical contacts, either (a)         photons are generated due to the recombination of free carriers         or (b) free-carrier pairs are generated due to the absorption of         photons. Examples of optically active devices include, without         limitation, lasers, optical amplifiers, optical modulators         (e.g., electroabsorption modulators, phase modulators, etc.),         variable optical attenuators, photodetectors, and the like. It         should be noted that an optically active device does not require         inclusion of a quantum-element-containing layer.     -   quantum element is defined as a semiconductor structure that         exhibits a quantum effect. Examples of quantum elements include,         without limitation, quantum dots, quantum wells, quantum-well         layers, quantum dashes, quantum wires, and the like.     -   passive waveguide is defined as a surface waveguide in which         light passes through virtually unperturbed. Passive waveguides         are not operatively coupled with electrical contacts and are not         stimulated to exhibit optoelectronic effects, such conversion of         free carriers into photons or vice versa, optical modulation,         optical amplification, and the like.

For the purposes of this Specification, including the appended claims, the terms “lateral” and “vertical” as used are meant to be relative to the major surfaces of a substrate on which an integrated-optics system resides, where the term lateral refers to directions that are parallel to the major surfaces and the term vertical refers to directions that are normal to the major surfaces. In similar fashion, the term “lower” means more proximate to the substrate and the term “higher” means more distal from the substrate.

FIG. 1 depicts a schematic drawing of a top view of an illustrative integrated-optics system in accordance with the present disclosure. System 100 includes active region 102, transition region 104, and output-coupling region 106, which are disposed on substrate 108. Active region 102, transition region 104, and output-coupling region 106 are contiguous regions that collectively define an integrated-optics-based, silicon-waveguide-coupled laser. System 100 comprises optically active (OA) device 110, coupling waveguide 112, passive waveguide 114, and silicon waveguide 116, which are arranged such that optical energy generated by OA device 110 propagates as light signal 118 from the laser to port 120.

It should be noted that, although the illustrative embodiment is an integrated-optics-based, silicon-waveguide-coupled laser wherein light is generated in an optically active device and propagates to a waveguide port, embodiments in accordance with the present disclosure include systems wherein a light signal is coupled into a waveguide port and conveyed to an OA device. Furthermore, in some embodiments, no silicon waveguide is included and port 120 is located in passive waveguide 114. Furthermore, in some embodiments, more than one passive waveguide and/or silicon waveguide is included.

FIGS. 2A-E depict schematic drawings of sectional views of system 100 through lines a-a through e-e, respectively.

FIG. 3 depicts operations of a method suitable for forming system 100 in accordance with the illustrative embodiment. Method 300 is described with continuing reference to FIGS. 1 and 2A-E. Method 300 begins with operation 301, wherein silicon waveguide 116 is defined on substrate 108 in output-coupling region 106.

Substrate 108 is a conventional silicon-on-insulator (SOI) substrate comprising handle substrate 202, buried oxide layer (BOX) 204, and silicon device layer 206. In the depicted example, handle substrate 202 is a conventional silicon wafer, BOX 204 is a layer of thermally grown silicon dioxide having a thickness that is typically within the range of approximately 1-2 microns and, preferably, 2 microns, and silicon device layer 206 is a layer of single-crystal silicon having a thickness equal to approximately 220 nm. As will be apparent to one skilled in the art after reading this Specification, however, substrate 108 can be any substrate suitable for use in system 100. Examples of substrates suitable for use in accordance with the present disclosure include, without limitation, glass substrates, compound-semiconductor substrates, bulk silicon substrates, and the like.

Silicon waveguide 116 is a rib waveguide having a rib portion of width w6. In output-coupling region 106, silicon waveguide is configured enable the waveguide to support single-mode propagation of light signal 118. Silicon waveguide 116 is formed by patterning silicon device layer 206 via conventional lithography and etching to define its structure. In some embodiments, silicon waveguide 116 has a waveguide structure other than that of a rib waveguide, such as a channel waveguide, strip waveguide, ridge waveguide, etc.

At operation 302, coupling layer 208 and active-material stack 210 are added to substrate 108.

In the depicted example, heterogeneous integration techniques are used to add coupling layer 208 and active-material stack 210 to substrate 108. Heterogeneous integration techniques suitable for use in accordance with the present disclosure are described in, for example, U.S. Pat. Nos. 9,097,848, 9,910,220, 8,380,033, 8,620,164, each of which is incorporated herein by reference in their entirety.

In accordance with conventional heterogeneous integration, a separate photonic substrate is formed by epitaxially growing active-material stack 210 and coupling layer 208 on a sacrificial substrate.

FIG. 4A depicts a schematic drawing of a cross-sectional view of a portion of a photonic substrate in accordance with the illustrative embodiment. Photonic substrate 400 includes coupling layer 208 disposed on active-material stack 210, which is disposed on sacrificial substrate 402.

In the depicted example, sacrificial substrate 402 is a conventional gallium arsenide wafer and coupling layer 208 is a layer of gallium arsenide having a thickness suitable for supporting single-mode propagation of light signal 118. In some embodiments, at least one of sacrificial substrate 402 and coupling layer 208 comprises a compound semiconductor other than gallium arsenide, such as indium phosphide, indium gallium arsenide, indium gallium arsenide phosphide, and the like.

Preferably, coupling layer 208 comprises sub-layers 208A and 208B, where sub-layer 208A has a lower refractive index than that of sub-layer 208B. As a result, sub-layer 208A can function as a cladding layer that serves to substantially confine at least a portion of the optical mode of light signal 118 to sub-layer 208B as the light signal propagates through passive waveguide 114. In some embodiments, coupling layer 208 includes at least one additional sub-layer distal to sub-layer 208A, where this additional sub-layer or sub-layers are configured to function as an upper cladding for sub-layer 208B. In some embodiments, coupling layer 208 does not include sub-layers (i.e., it is a homogeneous layer).

Furthermore, in some embodiments, sub-layer 208A is doped to reduce contact resistance for contacts 122 n.

Active-material stack 210 includes the constituent layers of OA device 110, including cladding layers, carrier confinement layers, and gain layer 212. In the depicted example, OA device 110 is a quantum-dot laser comprising active-material stack 210. In some embodiments, OA device 110 is a different optically active device, such as an optical modulator (e.g., an electroabsorption modulator, a phase modulator, etc.), an optical amplifier, a variable optical attenuator, a photodetector, and the like.

It should be noted that, although the illustrative embodiment includes a gain layer comprising a plurality of quantum dots, gain layer 212 can include any one or more of a wide variety of quantum elements without departing from the scope of the present disclosure. Quantum elements suitable for inclusion in gain layer 212 include, without limitation, quantum wells, quantum-well layers, quantum wires, quantum dashes, and the like.

In addition, as discussed below, preferably, active-material stack 210 also includes sub-layers having different refractive indices to control the vertical position of optical mode 216. For example, in some embodiments, active-material stack 210 contains higher aluminum content with gallium arsenide in the layers between active layer 212 and its top contact layer. This provides a lower index of refraction that can force optical energy in an optical mode within the material stack downward toward coupling waveguide 112. Furthermore, defining the active-material stack as a ridge or rib also helps force the optical mode toward the coupling waveguide while serving to laterally contain the optical mode as well.

In some embodiments, a dielectric layer is included between coupling layer 208 and silicon device layer 206 to act as a lower cladding that confines the optical mode toward at least one of the middle portion of passive waveguide 114 and the middle portion of active-material stack 210.

In some embodiments, active-material stack 210 also includes a region between active layer 212 and coupling waveguide 112 that has higher aluminum content to create a lower index of refraction, thereby forcing the optical mode upward away from the coupling waveguide. It should be noted that this same layer can also function as an upper cladding for coupling waveguide 112.

Once active-material stack 210 and coupling layer 208 are complete, photonic substrate 400 is then flipped over and coupling layer 208 is bonded to silicon device layer 206 via direct bonding. In some embodiments, a different bonding technology is used to join coupling layer 208 and device layer 206, such as plasma bonding, fusion bonding, thermo-anodic bonding, and the like. In some embodiments, an interface layer is included between silicon device layer 206 and coupling layer 208 to facilitate their bonding.

Once photonic substrate 400 and substrate 108 are bonded, sacrificial substrate 402 is removed in conventional fashion.

FIG. 4B depicts a schematic drawing of a cross-sectional view of nascent system 100′ after the removal of sacrificial substrate 402.

Although in the depicted example, coupling layer 208 and active-material stack 210 are added to substrate 108 via heterogeneous bonding techniques, in some embodiments, they are epitaxially grown on the substrate. In such embodiments, coupling layer 208 is grown directly on silicon device layer 206 via hetero-epitaxial growth, which is followed by epitaxial growth of active-material stack 210 on the coupling layer.

Returning now to method 300, at operation 303, active-material stack 210 is patterned to define the lateral dimensions of OA device 110. It should be noted that this removes active material completely from each of transition region 104 and output-coupling region 106. As a result, optically active material is selectively included in active region 102. However, as will be apparent to one skilled in the art, after reading this Specification, in some embodiments, a substrate has multiple active regions, each containing corresponding patterns of active-material stack 210.

OA device 110 is patterned such that it has nominal width w1 outside of the area of coupler 128-1, where the value of w1 is selected to facilitate lateral confinement of light signal 118 in the active region. In the depicted example, w1 is equal to 2 microns; however, w1 can have any suitable value. Typically, w1 is within the range of approximately 0.5 micron to approximately 4 microns.

At operation 304, coupling layer 208 is patterned in conventional fashion to define coupling waveguide 112 in active region 102 and passive waveguide 114 in transition region 104, where the passive waveguide supports single-mode propagation of light signal 118. It should be noted, however, that in active region 102, coupling waveguide 112 is not typically a single-mode waveguide for light signal 118.

Coupling waveguide 112 is formed as a rib waveguide. Coupling waveguide 112 includes a central ridge portion having width w2 and a planar portion having width w3. In the depicted example, w2 and w3 are 6 microns and 100 microns, respectively; however, each of w2 and w3 can have any suitable value. Typically, the value of w2 is within the range of approximately 0.5 micron to 20 microns. As will be apparent to one skilled in the art, the value of w3 is not typically critical, but is normally within the range of 25 to 500 microns.

It should be noted that, after operation 304, active-material stack 210 and coupling waveguide 112 collectively define composite waveguide 214 as a rib waveguide (also sometimes referred to as a strip waveguide), where the active-material stack functions as the projecting ridge portion of the composite waveguide and the central ridge portion of coupling waveguide 112 functions as the planar portion of the rib waveguide.

In the depicted example, the width, w2, of coupling waveguide 112 is selected such that the coupling waveguide, itself, contributes little or no lateral confinement of optical mode 216 within active region 102. As a result, the shape and size of optical mode 216 in active region 102 is determined primarily by the lateral dimensions of active material stack 210. In some embodiments, however, coupling waveguide 112 has a width that enables the coupling waveguide to provide lateral confinement of the optical mode. In some embodiments, the width of coupling waveguide 112 is substantially equal to the width of active-material stack 210 (i.e., w2=w1 and the coupling waveguide is substantially a channel waveguide).

Passive waveguide 114 is also formed as a rib waveguide comprising a central ridge portion having width w4 and a planar region having width w5. In the depicted example, w4 and w5 are 2 microns and 10 microns, respectively; however, each of w4 and w5 can have any suitable value. Typically, the value of w4 is within the range of approximately 0.5 micron to 4 microns. As will be apparent to one skilled in the art, the value of w5 is not typically critical, but is normally within the range of 0.5 to 100 microns.

Since coupling waveguide 112 and passive waveguide 114 are continuous segments of coupling layer 208, they are inherently optically coupled.

At operation 304, contacts 122 n are formed outside the central ridge portion of coupling waveguide 112.

At operation 305, contact 122 p is formed on the top surface of OA device 110 to complete the formation of system 100.

It should be noted that, in some embodiments, coupling waveguide is a slab waveguide (i.e., no ridge and planar portions are defined in coupling layer 208 within active region 102). In such embodiments, contacts 122 n can be formed on the top surface of the coupling layer, in vias partially etched down to sub-layer 208A, or in any manner suitable for making them operatively coupled with OA device 110. In some embodiments, coupling waveguide 112 is formed as a channel waveguide (i.e., no planar portion remains after coupling layer 208 has been etched to define the waveguide). In such embodiments, device layer is in electrical contact with sub-layer 208A and contacts 122 n are formed in silicon device layer 206.

Upon completion of system 100, active region 102 selectively includes OA device 110 and coupling waveguide 112, output-coupling region 106 selectively includes silicon waveguide 116, and transition region 104 includes passive waveguide 114. Passive waveguide 114 also extends into active region 102 to form coupler 128-1 with coupling waveguide 112 and extends into output-coupling region 106 to form coupler 128-2 with silicon waveguide 116.

It is an aspect of the present invention that the lateral dimensions of active-material stack 210 substantially determine the vertical position at which optical energy in OA device 110 forms an optical mode, as well as the shape of that optical mode. As a result, active material stack 210 is defined such that it includes a first segment (i.e., gain section 124) that is configured to favor optical gain that gives rise to an optical mode and a second segment (i.e., taper 126-1) that is configured to force that optical mode into coupling waveguide 112.

FIG. 2A depicts a schematic drawing of a sectional view of OA device 110 taken through gain section 124 (i.e., through line a-a shown in FIG. 1).

As seen in FIG. 2A, in gain section 124, the relative values of w1 and w2 give rise to optical mode 216 such that its optical energy is contained within one continuous region that is substantially centrally located in composite waveguide 214. In other words, each of OA device 110 and coupling waveguide 112 partially supports optical mode 216.

In some embodiments, coupling waveguide 112 and active-material stack 210 are configured such that optical mode 216 extends over a continuous region that includes at least a portion of each of the active-material stack, the coupling waveguide, and silicon device layer 206 (i.e., optical mode 216 is partially supported by each of the active-material stack, the coupling waveguide, and the silicon device layer).

Coupler 128-1 is a section of active region 102 configured for forcing substantially all of the optical energy of light signal 118 located in active-material stack 210 into coupling waveguide 112 so that it can efficiently couple into passive waveguide 114. Coupler 128-1 includes tapers 126-1 and 126-2.

Taper 126-1 is a segment of active-material stack 210 that is configured to force the optical energy of light signal 118 into coupling waveguide 112. Taper 126-1 has length L1 and a width that reduces from w1 to zero (i.e., extinction) along length L1. In the depicted example, L1 is equal to 100 microns; however, it is typically within the range of approximately 50 microns to approximately 500 microns. It should be noted that the value of L1 is a matter of design choice and, therefore, it can have any suitable value. Furthermore, in some embodiments, taper 126-1 does not taper to extinction but, rather, to a non-zero width (e.g., one micron or less) that is sufficiently narrow to force the optical energy of light signal 118 from OA device 110 into passive waveguide 114.

In similar fashion, taper 126-2 is a segment of coupling waveguide 112 that is configured to facilitate the transfer of light signal 118 into passive waveguide 114 as a single-mode signal. Taper 126-2 has length L2, over which the width of coupling waveguide 112 changes from w2 to w4 (i.e., the width of passive waveguide 114). In the depicted example, L2 is equal to 50 microns; however, it is typically within the range of approximately 10 microns to approximately 500 microns. It should be noted that the value of L2 is not critical and, therefore, it can have any value within a wide range.

FIG. 2B depicts a schematic drawing of a sectional view of coupler 128-1 as taken through line b-b shown in FIG. 1.

As noted above, the distribution of the optical energy of optical mode 216 between active-material stack 210 and coupling layer 208 (i.e., the shape and vertical position of the optical mode) is based on the relationship of w1 and w2, which change along length L1 of taper 126-1. Tapers 126-1 and 126-2 are configured, therefore, to force optical mode 216 substantially completely into coupling waveguide 112 by the time light signal 118 reaches transition region 104.

In similar fashion, in the depicted example, w4 and w5 of passive waveguide 114 are selected such that optical mode 216 is substantially confined within its ridge and planar regions within transition region 104. In some embodiments, however, passive waveguide 114 is configured such that the optical energy of optical mode 216 extends across a continuous region that occupies at least portions of both passive waveguide 114 and silicon device layer 206. In other words, each of passive waveguide 114 and silicon device layer 206 partially supports optical mode 216.

FIG. 2C depicts a schematic drawing of a sectional view of transition region 104 as taken through line c-c shown in FIG. 1.

Coupler 128-2 is a section of output-coupling region 106 configured for efficiently optically coupling light signal 118 from passive waveguide 114 into silicon waveguide 116. Coupler 128-2 includes taper 126-3 and a segment of silicon waveguide 116, where taper 126-3 is a segment of passive waveguide 114 that is configured to facilitate the transfer of light signal 118 into silicon waveguide 116 as a single-mode signal.

FIG. 2D depicts a schematic drawing of a sectional view of coupler 128-2 as taken through line d-d shown in FIG. 1. Note that the planar portion of silicon waveguide 116 is not shown in FIG. 1.

Taper 126-3 has length L3, over which the width of passive waveguide 114 changes from w4 to extinction. As a result, taper 126-3 is configured to force optical mode 216 completely into silicon waveguide 116. In the depicted example, L3 is equal to 200 microns; however, it is typically within the range of approximately 50 microns to approximately 1000 microns. It should be noted that the value of L3 is a matter of design choice and, therefore, it can have any suitable value. As discussed above and with respect to taper 126-1, in some embodiments, taper 126-3 does not taper to extinction but, rather, to a non-zero width (e.g., one micron or less) that is sufficiently narrow to force the optical energy of light signal 118 from passive waveguide 114 into silicon waveguide 116. Furthermore, in some embodiments, silicon waveguide 116 includes a taper that facilitates transfer of optical energy between the silicon and passive waveguides. In some embodiments, both passive waveguide 114 and silicon waveguide 116 include a taper.

FIG. 2E depicts a schematic drawing of a sectional view of silicon waveguide 116 as taken through line e-e shown in FIG. 1. It should be noted that, in the depicted example, the regions between silicon waveguide 116 and taper 126-3 are air-cladding regions; however, these regions can be filled with any material suitable to function as cladding material for the silicon waveguide, such as silicon dioxide, silicon nitride, polymer, and the like.

It is another aspect of the present invention that OA device 110 can be configured to support an optical mode that is discontinuous such that, at some points within active region 102, it includes separate optical-mode portions that propagate together but are distributed among active-material stack 210 and coupling waveguide 114 and, in some embodiments, silicon device layer 206. In other words, each of active-material stack 210 and coupling waveguide 114 and silicon device layer 206 partially supports optical mode 216 by supporting a different one of its optical-mode portions.

FIG. 5A depicts a schematic drawing of a sectional view of an alternative composite waveguide in accordance with the present disclosure. The sectional view depicted in FIG. 5A is taken through a region analogous to that intersected by line a-a shown in FIG. 1. Composite waveguide 500 includes substrate 108, coupling waveguide 502, and active-material stack 504. For clarity, electrical contacts are not shown in FIG. 5A.

In composite waveguide 500, coupling waveguide 502 and active-material stack 504 are configured such that each includes at least one sub-layer that is configured to force optical energy of optical mode 216 into a different sub-layer of that element. As a result, optical mode 216 is split into two discontinuous optical-mode portions—optical-mode portions 216A and 216B. In other words, each of coupling waveguide 502 and active-material stack 504 partially supports optical mode 216 by supporting a different one of optical-mode portions 216A and 216B.

Coupling waveguide 502 is analogous to coupling waveguide 112; however, coupling waveguide 502 has width w8 and includes sub-layers 502A, 502B, and 502C, where each of sub-layers 502A and 502C has a refractive index that is lower than that of sub-layer 502B. As a result, sub-layers 502A and 502C function as lower and upper cladding layers, respectively, that substantially confine the bulk of the optical energy of optical-mode portion 216A to sub-layer 502B.

Active-material stack 504 is analogous to active-material stack 210; however, active-material stack 504 has width w7 and includes sub-layers 506A and 506B, each of which has a refractive index that is higher than that of gain layer 212. As a result, sub-layers 506A and 506B function as upper and lower cladding layers, respectively, that substantially confine the bulk of the optical energy of optical-mode portion 216B to the portion of active-material stack 504 that resides between them.

The widths of optical-mode portions 216A and 216B and the spacing between them are based upon widths w7 and w8 and the sub-layer configurations of coupling waveguide 502 and active-material stack 504.

In some embodiments, coupling waveguide 502 and active-material stack 504 are configured such that optical mode 216 includes a third optical-mode portion that is located in silicon device layer 206.

FIG. 5B depicts a schematic drawing of a sectional view of another alternative composite waveguide in accordance with the present disclosure. The sectional view depicted in FIG. 5B is taken through a region analogous to that intersected by line a-a shown in FIG. 1. Composite waveguide 508 is analogous to composite waveguide 500; however, in composite waveguide 508, coupling waveguide 502 and active-material stack 504 are configured to split the optical energy of optical mode 216 into three discontinuous optical-mode portions—optical-mode portions 216C, 216D, and 216E.

Coupling waveguide 510 is analogous to coupling waveguide 502; however, coupling waveguide 510 has width w10 and includes sub-layers 502D, 502E, and 502F, where each of sub-layers 502D and 502F has a refractive index that is lower than that of sub-layer 502E. As a result, sub-layers 502D and 502F function as lower and upper cladding layers, respectively, that substantially confine the bulk of the optical energy of optical-mode portion 216D to sub-layer 502E.

Active-material stack 512 is analogous to active-material stack 504; however, active-material stack 512 has width w9 and includes sub-layers 514A and 514B, each of which has a refractive index that is lower than that of gain layer 212. As a result, sub-layers 514A and 514B function as upper and lower cladding layers, respectively, that substantially confine the bulk of the optical energy of optical-mode portion 216C to the portion of active-material stack 512 that resides between them.

In the depicted example, coupling waveguide 510 and active-material stack 512 are further configured to give rise to additional optical-mode portion 216E, which is discontinuous with optical-mode portions 216C and 216D and substantially confined to silicon device layer 206.

As a result, each of coupling waveguide 510, active-material stack 512, and silicon device layer 206 partially supports optical mode 216 by supporting a different one of its optical-mode portions. The widths of optical-mode portions 216C, 216D, and 216E, as well as the spacing between them, are based upon widths w9 and w10 and the sub-layer configurations of coupling waveguide 510 and active-material stack 512.

In some embodiments, coupling layer 208 is configured such that optical mode 216 is split into discontinuous optical-mode portions in transition region 104, with one of the optical-mode portions being located in passive waveguide 112 and another optical-mode portion is located in silicon device layer 206.

FIG. 6 depicts a schematic drawing of a sectional view of an alternative transition region in accordance with the present disclosure. The sectional view depicted in FIG. 6 is taken through a region analogous to that intersected by line c-c shown in FIG. 1. Transition region 600 includes substrate 108, passive waveguide 602, and cladding layer 604. Transition region 600 is analogous to transition region 104 described above; however, in transition region 600, optical mode 216 includes discontinuous optical-mode portions 216E and 216F, which reside in passive waveguide 602 and silicon device layer 206, respectively.

Passive waveguide 602 is analogous to passive waveguide 114; however, passive waveguide 602 includes sub-layer 606, which has a refractive index that is higher than the remainder of the passive waveguide.

Cladding 604 is a thin layer of material suitable for substantially blocking the passage of optical energy of light signal 118 between passive waveguide 114 and silicon device layer 206. In the depicted example, cladding 604 is a layer of silicon dioxide having a thickness of approximately 100 nm; however, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments wherein cladding 604 comprises a different material and/or has a different thickness.

As noted above, passive waveguide 602, cladding 604 and sub-layer 606 are configured such that they collectively support optical mode 216, which includes discontinuous optical-mode portions 216F and 216G, which are located in passive waveguide 114 and silicon device layer 206, respectively (i.e., passive waveguide 114 partially supports optical mode 216 by supporting optical-mode portion 216F and silicon device layer 206 partially supports optical mode 216 by supporting optical-mode portion 216G). The shapes, vertical positions, and separation between optical-mode portions 216F and 216G are based on the values of w11 that location.

FIG. 7 depicts a schematic drawing of a sectional view of an alternative embodiment of a coupler for optically coupling a passive waveguide and a silicon waveguide in accordance with the present disclosure. The sectional view depicted in FIG. 7 is taken through a region analogous to that intersected by line d-d shown in FIG. 1. Coupler 700 includes substrate 108, silicon waveguide 116, cladding 702, and taper 704.

Cladding 702 is analogous to cladding 604.

Taper 704 is analogous to taper 126-3; however, taper 704 has width w12(x) along length L3 and includes sub-layer 706, which has a refractive index that is higher than the remainder of the taper. As a result, sub-layer 706 serves to confine optical energy of light signal 118 to the region of taper 704 located between cladding 702 and sub-layer 706.

By virtue of cladding 702 and sub-layer 706, optical mode 216 includes discontinuous optical-mode portions 216H and 216J, which are located in and supported by silicon waveguide 116 and taper 704, respectively. As a result, each of silicon waveguide 116 and taper 704 partially supports optical mode 216. The shapes, vertical positions, and separation between optical-mode portions 216H and 216J at any location along length L3 are based on the values of w12(x) and w6 at that location and the configuration of taper 704 and sub-layer 706.

In some embodiments, light signal 118 is optically coupled between passive waveguide 114 and silicon waveguide 116 via a turning reflector and vertical grating coupler.

FIG. 8 depicts a schematic drawing of a cross-sectional view of yet another alternative coupler in accordance with the present disclosure. Coupler 800 includes reflector 802 and vertical grating coupler 804.

Reflector 802 is an angled facet formed in passive waveguide 114. Reflector 802 is configured to receive light signal 118 propagating along longitudinal axis A1 of the passive waveguide and redirect it along a direction that is substantially normal to axis A1. In some embodiments, reflector 802 includes one or more surface layers (e.g., metals, dielectrics, etc.) for improving its reflectivity for light signal 118.

Grating 804 is a vertical grating coupler formed in silicon waveguide 116 and configured to receive light signal 118 from reflector 802 and redirect it along longitudinal axis A2 of the silicon waveguide.

As will be apparent to one skilled in the art, a vertical grating coupler typically has a range of angles at which light signal 118 can be received and successfully coupled into a waveguide. As a result, reflector 802 can be configured to redirect the light signal along any angle within the acceptance range of grating 804.

In some embodiments, grating 804 is formed in passive waveguide 114 and reflector 802 is formed in silicon waveguide 116.

In some embodiments, silicon waveguide includes an output port comprising a reflector configured to launch light signal 118 out of the plane of the silicon waveguide as a free-space signal.

FIGS. 9A and 9B depict schematic drawings of cross-sectional views of alternative output ports in accordance with the present disclosure. Each of ports 900 and 902 include reflector 904, which is formed in silicon waveguide 116.

Reflector 904 is analogous to reflector 802 described above; however, reflector 904 is formed in silicon waveguide 116 to redirect light signal 118 as indicated.

Port 900 includes reflector 904 and facet 906. Facet 906 is formed in silicon waveguide 116 such that light signal 118 is launched into free space and received by reflector 904. Reflector 904 is configured to redirect free-space light signal 118 away from handle substrate 202.

Port 902 includes reflector 904 is configured to launch light signal 118 as a free-space signal directed toward substrate 908.

Substrate 908 is analogous to substrate 108 described above; however, it is preferable that substrate 908 comprise a material that is substantially transparent and non-absorptive for the wavelengths of light signal 118 so that the light signal can pass completely through the substrate with little or no attenuation.

While it is preferable in most applications to optically couple light signal 118 between OA device 110 and silicon waveguide 116, in some embodiments, the light signal is provided to, or received from, an external device or system without being coupled into the silicon waveguide.

FIG. 10 depicts a schematic drawing of a cross-sectional view of an alternative system in accordance with the present disclosure. System 1000 is analogous to system 100; however, in system 1000, light signal 118 is launched into free space directly from passive waveguide 114. System 1000 is disposed on substrate 1002 and includes facet 1004 and bulk reflector 1006.

Substrate 1002 is a bulk silicon substrate suitable for use in planar processing.

Facet 1004 is an end facet formed in passive waveguide 114 in conventional fashion (e.g., by etching, dicing, partial dicing, etc.). At facet 1004, light signal 118 exits the passive waveguide as free-space signal 1008.

Reflector 1006 is analogous to reflector 804; however, reflector 1006 is a bulk reflector mounted on substrate 1002 such that it receives free-space signal 1008 from passive waveguide 114. In some embodiments, reflector 1006 is formed in a region of coupling layer 208 outside of the area of passive waveguide 114.

In some embodiments, it is preferable to precisely locate a bulk optical element to receive free-space signal 1008 directly from passive waveguide 114.

FIG. 11 depicts a schematic drawing of a cross-sectional view of another alternative system in accordance with the present disclosure. System 1100 is analogous to system 1000; however, system 1100 includes bulk optical element 1102 and alignment feature 1104. Element 1102 is precisely located on substrate 1002 by alignment feature 1104 such that it receives light signal 118 from passive waveguide 114.

In the depicted example, element 1102 is an optical fiber; however, element 1102 can include a wide range of device and systems without departing from the scope of the present disclosure. Devices and systems suitable for use in element 1102 include, without limitation, optical fibers, PICs, photodetectors, light sources (edge-emitting lasers, vertical-cavity surface-emitting lasers (VCSELs), light-emitting diodes, integrated-optics systems, planar-lightwave circuits (PLCs), and the like.

Alignment feature 1104 is a channel etched in substrate 1002 via conventional methods (e.g., reactive-ion etching, crystallographic-dependent etching, ion milling, laser-assisted etching, etc.) such that its depth aligns the core of element 1102 with passive waveguide 114. In some embodiments, alignment feature 1104 includes at least one projection disposed on the top surface of substrate 1002 (or silicon device layer 206 in embodiments where system 1100 is disposed on an SOI, such as substrate 108), where the projection is configured to constrain element 1102 in at least one dimension.

It is to be understood that the disclosure teaches only examples of embodiment in accordance with the present disclosures and that many variations of these embodiments can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims. 

What is claimed is:
 1. An integrated-optics system disposed on a substrate, the system comprising an optically active device that is optically coupled with a silicon waveguide, the system including: a first region that selectively includes the optically active device comprising an active-material stack the active-material stack being disposed on a coupling waveguide that includes a first segment of a first layer that comprises a compound semiconductor, wherein the optically active device and the coupling waveguide collectively define a composite waveguide that at least partially supports an optical mode of a light signal; a second region that includes a first waveguide that is a passive waveguide configured to at least partially support the optical mode, the first waveguide having a first core that comprises a second segment of the first layer, the first waveguide being optically coupled with the composite waveguide; and a third region that selectively includes a second waveguide having a second core comprising single-crystal silicon, wherein the second waveguide is configured to at least partially support the optical mode and is optically coupled with the first waveguide; wherein the first region, second region, and third region are contiguous.
 2. The system of claim 1 wherein, in the first region, the coupling waveguide is disposed on a second layer comprising single-crystal silicon, and wherein the optical mode extends over a continuous region that includes at least a portion of each of the active-material stack, the coupling waveguide, and the second layer.
 3. The system of claim 1 wherein, in the second region, the passive waveguide is disposed on a second layer comprising single-crystal silicon, and wherein the optical mode extends over a continuous region that includes at least a portion of each of the passive waveguide and the second layer.
 4. The system of claim 1 wherein, in the second region, the passive waveguide is disposed on a second layer comprising single-crystal silicon, and wherein the optical mode includes a first optical-mode portion and a second optical-mode portion, the first and second optical-mode portions being discontinuous, and wherein the first optical-mode portion is located in the passive waveguide and the second optical-mode portion is located in the second layer.
 5. The system of claim 1 wherein, in the first region, the optical mode includes a first optical-mode portion and a second optical-mode portion, the first and second optical-mode portions being discontinuous, and wherein the first optical-mode portion is located in the optically active device and the second optical-mode portion is located in the coupling waveguide.
 6. The system of claim 5 wherein, in the first region, the coupling waveguide is disposed on a second layer that comprises single-crystal silicon and the optical mode further includes a third optical-mode portion that is located in the second layer, and wherein the first, second, and third optical-mode portions are discontinuous.
 7. The system of claim 6 wherein the substrate is a silicon-on-insulator substrate that includes a handle substrate, a buried-oxide layer, and a device layer, and wherein the second layer is the device layer, and further wherein the second core comprises at least a portion of the second layer.
 8. The system of claim 1 wherein the optically active device includes at least one quantum element, and wherein the at least one quantum element is selected from the group consisting of a quantum dot, a quantum well, a quantum-well layer, a quantum dash, and a quantum wire.
 9. The system of claim 1 wherein the optically active device includes a first taper having a first length and a lateral dimension that changes from a first width to a second width along the first length, and wherein at least one of the size and the vertical position of the optical mode at each location along the first length is based on the lateral dimension at that location.
 10. The method of claim 9 wherein the second width is equal to zero.
 11. The system of claim 1 further comprising a first reflector that is configured to receive the light signal along a first direction and redirect the light signal along a second direction that is unaligned with the first direction.
 12. The system of claim 11 further comprising a vertical-grating coupler configured to receive the light signal from the first reflector, the first reflector being located in the first waveguide and the vertical grating coupler being located in the second waveguide.
 13. The system of claim 11 wherein the first reflector is located in one of the first waveguide and the second waveguide.
 14. The system of claim 11 wherein the first reflector is configured to receive the light signal from one of the first waveguide and second waveguide as a free-space light signal.
 15. The system of claim 1 further comprising a first element and a first alignment feature for positioning the first element relative to one of the first waveguide and the second waveguide such that the first element and the OA device are optically coupled, wherein the first element is selected from the group consisting of an optical fiber, a surface waveguide, an optical detector, a laser, an optical amplifier, and a light-emitting diode.
 16. The system of claim 1 wherein the optically active device is selected from the group consisting of a laser, an optical modulator, an optical amplifier, a variable optical attenuator, and a photodetector.
 17. The system of claim 1 wherein the passive waveguide includes a first taper having a first length and a lateral dimension that changes from a first width to a second width along the first length, and wherein at least one of the size and the vertical position of the optical mode at each location along the first length is based on the lateral dimension at that location.
 18. The method of claim 17 wherein the second width is equal to zero.
 19. The system of claim 1 wherein the first layer has a first sub-layer that has a first refractive index and a second sub-layer that has a second refractive index that is higher than the first refractive index, the second sub-layer being between the first sub-layer and the optically active device, and wherein the optically active device includes a third sub-layer that has a third refractive index that is lower than the second refractive index.
 20. A method for forming an integrated-optics system disposed on a first substrate, the system having a first region, a second region, and a third region and comprising an optically active device that is optically coupled with a silicon waveguide, the method including: forming a coupling waveguide in the first region, the coupling waveguide including a first segment of a first layer comprising a compound semiconductor; forming an optically active device that includes an active-material stack, the active-material stack being disposed on the coupling waveguide, wherein the optically active device and the coupling waveguide collectively define a first composite waveguide that at least partially supports an optical mode of a light signal, and wherein the optically active device is located only in the first region; forming a first waveguide in a second region, the first waveguide being a passive waveguide configured to at least partially support the optical mode, wherein the first waveguide has a first core that comprises a second segment of the first layer, and wherein the first waveguide is optically coupled with the first composite waveguide; and forming a second waveguide in a third region, the second waveguide having a second core comprising single-crystal silicon and being configured to at least partially support the optical mode, wherein the second waveguide is optically coupled with the first waveguide.
 21. The method of claim 20 wherein, in the first region, the optical mode includes a first optical-mode portion and a second optical-mode portion, the first and second optical-mode portions being discontinuous, and wherein the optically active device and the coupling waveguide are formed such that the first optical-mode portion is located in the optically active device and the second optical-mode portion is located in the coupling waveguide.
 22. The method of claim 20 wherein, in the first region, the coupling waveguide is disposed on a second layer that comprises single-crystal silicon and the optical mode further includes a third optical-mode portion that is located in the second layer, and wherein the first, second, and third optical-mode portions are discontinuous.
 23. The method of claim 20 wherein the first substrate is provided as a silicon-on-insulator substrate that includes a handle substrate, a buried-oxide layer, and a device layer, and wherein the second layer is the device layer, and further wherein the second waveguide is formed such that the second core comprises at least a segment of the second layer.
 24. The method of claim 20 wherein, in the first region, the coupling waveguide is disposed on a second layer comprising single-crystal silicon, and wherein the optical mode extends over a continuous region that includes at least a portion of each of the optically active device, the coupling waveguide, and the second layer.
 25. The method of claim 20 wherein, in the second region, the passive waveguide is disposed on a second layer comprising single-crystal silicon, and wherein the optical mode extends over a continuous region that includes at least a portion of each of the passive waveguide and the second layer.
 26. The method of claim 20 wherein, in the second region, the passive waveguide is disposed on a second layer comprising single-crystal silicon, and wherein the optical mode includes a first optical-mode portion and a second optical-mode portion, the first and second optical-mode portions being discontinuous, and wherein the first optical-mode portion is located in the passive waveguide and the second optical-mode portion is located in the second layer.
 27. The method of claim 20 further comprising: epitaxially growing the first layer on a second layer that comprises single-crystal silicon, the first substrate including the second layer; epitaxially growing the active-material stack on the first layer; patterning the first layer and the active-material stack in the first region to define the first composite waveguide; and forming a plurality of electrical contacts that are operatively coupled with the active-material stack.
 28. The method of claim 20 wherein the optically active device is formed such that it includes at least one quantum element that is selected from the group consisting of a quantum dot, a quantum well, a quantum-well layer, a quantum dash, and a quantum wire.
 29. The method of claim 20 wherein the optically active device is selected from the group consisting of a laser, an optical modulator, an optical amplifier, a variable optical attenuator, and a photodetector.
 30. The method of claim 20 further comprising: epitaxially growing the first layer and the active-material stack on a second substrate; joining the first layer and a second layer, the first substrate comprising the second layer, wherein the second layer comprises a first material selected from the group consisting of single-crystal silicon, silicon oxides, silicon nitrides, polymers, and metals; patterning the first layer and the active-material stack in the first region to define the first composite waveguide; and forming a plurality of electrical contacts that are operatively coupled with the active-material stack.
 31. The method of claim 20 further comprising patterning the active-material stack in the first region such that the optically active device has a first taper having a first length and a first lateral dimension that changes from a first width to a second width along the first length, wherein at least one of the size and the vertical position of the optical mode in the first composite waveguide at each location along the first length is based on the first lateral dimension at that location.
 32. The method of claim 31 wherein the second width is equal to zero.
 33. The method of claim 20 further comprising forming a first reflector that is configured to receive the light signal along a first direction and redirect the light signal along a second direction that is unaligned with the first direction.
 34. The method of claim 33 further comprising forming a vertical grating coupler configured to receive the light signal from the first reflector, the first reflector being located in the first waveguide and the vertical grating coupler being located in the second waveguide.
 35. The method of claim 33 further comprising forming the first reflector such that it is located in one of the first waveguide and the second waveguide.
 36. The method of claim 33 further comprising forming the first reflector such that it is configured to receive the light signal from one of the first waveguide and second waveguide as a free-space light signal.
 37. The method of claim 20 further comprising forming a first alignment feature that is configured to vertically position a first element relative to one of the first waveguide and the second waveguide to enable the light signal to optically couple into the first element, wherein the first element is selected from the group consisting of an optical fiber, a surface waveguide, an optical detector, a laser, an optical amplifier, and a light-emitting diode.
 38. The method of claim 20 wherein the first region is formed such that the first layer has a first sub-layer that has a first refractive index and a second sub-layer that has a second refractive index that is higher than the first refractive index, the second sub-layer being between the first sub-layer and the optically active device, and wherein the optically active device includes a third sub-layer that has a third refractive index that is lower than the second refractive index.
 39. The method of claim 20 wherein forming the second region includes patterning the first layer to define the first core.
 40. The method of claim 20 wherein forming the third region includes operations comprising: patterning a second layer to define the second core, wherein the second layer comprises single-crystal silicon; and patterning the first layer to define a taper having a first length and a lateral dimension that changes from a first width to a second width along the first length, wherein the first layer and second layer are in contact along at least a portion of the first length; wherein at least one of the size and vertical position of the optical mode at each location along the first length is based on the lateral dimension at that location.
 41. The method of claim 40 wherein the second width is equal to zero. 